The new cultivar of Prunus domestica of the present invention was created during 1987 in the course of prune breeding research carried out at the Kearney Agricultural Center of the University of California located at Parlier, Calif. Prunus domestica is commonly known as the European plum. The female parent (i.e., seed plant) was the European plum cultivar xe2x80x98Sugarxe2x80x99 (non-patented in the United States) and the male parent (i.e., pollen parent) was the prune cultivar xe2x80x98Primacotesxe2x80x99 (non-patented in the United States). The parentage of the new cultivar is expressed as follows:
xe2x80x98Sugarxe2x80x99xc3x97xe2x80x98Primacotesxe2x80x99. 
During the course of the breeding program over 500 crosses were attempted following emasculation. Seeds resulting from such cross-pollination were harvested at the end of the growing season. These were planted during 1988 and the resulting plants were given the group designation P88.17. The seedlings were grown in a nursery at Parlier, Calif. and were carefully studied during the remainder of 1988 and 1989. At the end of the 1989 growing season, 205 small trees were dug and were placed in cold storage. These trees were transplanted into seedling rows in the spring of 1991 and their study continued. A single tree of the new cultivar of the present invention was selected during 1993 when such seedling first fruited. This seedling initially was designated 4-6W-53.
It was found that the new Prunus domestica cultivar of the present invention:
(a) Exhibits a vigorous growth habit,
(b) Is productive and regular bearing, and
(c) Forms dark purple fruit with a medium waxy bloom capable of being dried to a high quality prune that, when compared to the fruit of the xe2x80x98Improved Frenchxe2x80x99 cultivar, is larger, matures approximately 7 to 10 days earlier, and possesses a lighter, more complex fruity taste with a higher sugar content.
The new cultivar has been asexually reproduced by grafting and budding. During February of 1994, the new cultivar was first asexually propagated at Parlier, Calif. by grafting onto xe2x80x98Mariannaxe2x80x99 plum rootstock (non-patented in the United States). The resulting tree produced a small amount of fruit in 1995 and the first significant amount of fruit in 1996. Good fruit production continued in 1997, 1998, and 1999. The fruit produced on the propagated tree was the same as that of the original seedling in all respects. The new cultivar was first grafted onto xe2x80x98Myrobalan 29Cxe2x80x99 rootstock (non-patented in the United States) in 1996. Such propagation was also successful. Attempted field grafts onto peach rootstocks have resulted in scion breakage at the graft union. Accordingly, at the present time, peach rootstocks are not recommended. The new cultivar was found to reproduce true to form via such asexual propagation using xe2x80x98Mariannaxe2x80x99 and xe2x80x98Myrobalanxe2x80x99 rootstock and performed well on such rootstocks in all respects.
The new cultivar has been further evaluated at test sites in the San Joaquin and Sacramento Valleys of California. These tests have further confirmed the commercial potential of the new cultivar.
In a typical year, the fruit of new cultivar commonly reaches maturity during early to mid-August at Parlier, Calif. This is approximately 7 to 10 days earlier at such location than that produced on the most commonly grown xe2x80x98Improved Frenchxe2x80x99 prune cultivar (non-patented in the United States). Accordingly, the new cultivar can be grown to advantage in conjunction with the xe2x80x98Improved Frenchxe2x80x99 cultivar so that harvest and processing season at a given location can be extended. The fruit shape resembles that of the xe2x80x98Improved Frenchxe2x80x99 cultivar, but commonly is approximately 15 to 20 percent larger. Also, the fruit of the new cultivar commonly develops at least 2 degrees Brix soluble solids more than the xe2x80x98Improved Frenchxe2x80x99 cultivar at full maturity. Limited tests with mechanical harvesting of the fruit have been successful with very little fruit damage.
When the new cultivar of the present invention is compared to the xe2x80x98Sugarxe2x80x99 parent, there are distinct differences in areas of fruit shape, fruit color, fruit size, stone shape, stone size and adhesion, production characteristics, and tree form. The fruit of the xe2x80x98Sugarxe2x80x99 cultivar is oval in form and reddish to reddish-purple in coloration. In contrast the fruit of the new cultivar is pyriform in shape with a broad but distinct neck and is dark purple-red to dark purple-blue in coloration. The fruit of the xe2x80x98Sugarxe2x80x99 cultivar is medium in size and about 10 percent larger than the industry standard xe2x80x98Improved Frenchxe2x80x99 cultivar in an average to light-set year. The fruit of the new cultivar is medium to large in size and commonly 15 to 20 percent larger than that of the xe2x80x98Improved Frenchxe2x80x99 cultivar. The stone of the xe2x80x98Sugarxe2x80x99 prune is large and commonly has a large protruding wing along the basal suture area. This wing can lead to the presence of excessive pit fragments in mechanically pitted dried fruit. The stone of the new cultivar is smaller than that of the xe2x80x98Sugarxe2x80x99 cultivar and possesses no wing or other protrusion. The stone of the xe2x80x98Sugarxe2x80x99 cultivar also is a clingstone and tightly adheres to the fruit flesh. The xe2x80x98Sugarxe2x80x99 cultivar is decidedly alternate bearing. The new cultivar has little tendency to alternate bear. The trees of the xe2x80x98Sugarxe2x80x99 cultivar are very open in form with less tree density then either the xe2x80x98Improved Frenchxe2x80x99 cultivar or the new cultivar of the present invention. The date of maturity for the prunes of the xe2x80x98Sugarxe2x80x99 cultivar commonly ranges from approximately five days to a week before that of the presently claimed cultivar.
It will be further noted that substantial differences exist between the new cultivar and the xe2x80x98Primacotesxe2x80x99 parent with regard to fruit color, the distribution of fruit on the tree, the date of maturity, the soluble solids content of the fruit, tree productivity, and sensitivity to heat. The fruit of the xe2x80x98Primacotesxe2x80x99 cultivar is reddish to reddish-purple in coloration and is primarily borne in large clusters towards the ends of the previous season""s shoots. This reuslts in lower productivity. The fruit of the new cultivar is borne throughout the tree and as indicated is dark purple-red to dark purple-blue in coloration. In California the fruit of the xe2x80x98Primacotesxe2x80x99 cultivar commonly ripens approximately two weeks ahead of the xe2x80x98Improved Frenchxe2x80x99 cultivar. At full maturity the xe2x80x98Primacotesxe2x80x99 cultivar commonly develops a fruit soluble solids level of no more than 22 degrees Brix while that of the new cultivar commonly averages 24 Brix or above. When grown in the hot interior San Joaquin Valley of California, the xe2x80x98Primacotesxe2x80x99 cultivar often displays a moderate amount of internal heat damage to the fruit. Such heat damage has never been observed in the new cultivar under the same growing conditions.
The pollination requirements for the new cultivar are not fully known to date. Limited tests indicate that the variety is at least partially self-fruitful. Fruit set may be enhanced by the use of a pollinator. Limited tests indicate that the xe2x80x98Improved Frenchxe2x80x99 cultivar having a bloom date which well corresponds to that of the new cultivar can serve as a pollinator.
The disease resistance/susceptibility of the new cultivar based on field observations appears to be substantially the same as that of the xe2x80x98Improved Frenchxe2x80x99 cultivar during observations to date. Such observations have been made primarily with respect to Brown Rot, the principle fungal disease found in prunes in California.
The new cultivar of the present invention is particularly well-suited for the dried fruit market. The fruit stone is nearly free and machine pits well. Also, the new cultivar is a candidate for the fresh fruit market. Currently the fresh prune crop produced in the United States is largely exported to Pacific Rim countries where it is held in particularly high regard. The smaller size commonly exhibited by fruit sold in the fresh fruit market may limit this end use. In particularly heavy crop years where size is further diminished, the possibility of usage in the fresh fruit market likely will be further diminished.
The new cultivar has been named xe2x80x98Sutterxe2x80x99.